Welcome to the Daily TWiP, your daily dose of all the holidays, historical observances, etc., we couldn’t cram into The Week in Preview.

Today in 1948, a stifling yellow smog descended on the industrial mill town of Donora, Pa. It blanketed the town until Oct. 31st, when it was finally dispersed by rain. By then, 20 people had died of what was thought to be asthma and one-third to one half of the town’s 14,000 residents were suffering from respiratory problems.

Donora is located in the Monongahela River valley and was home to the Donora Zinc Works and the American Steel & Wire plant, both of which were operated by U.S. Steel. Sulfur dioxide emissions were not uncommon in the town, but this smog was an entirely different animal.

A temperature inversion occurred on Oct. 26th, trapping a mass of warm, stagnant air in the town. Because of this, the pollutants from the industrial plants didn’t disperse into the air like they normally did. Instead, they mixed with the warm air mass and continued to hang over the town.

For several days, Donora residents breathed in concentrated amounts of sulfuric acid, nitrogen dioxide, fluorine, and other lovely pollutants. Autopsies performed on the 20 individuals who died during the smog revealed that levels of fluorine in their bodies were well into the lethal range, making fluorine poisoning, not asthma, their primary cause of death.

Fluorine gas is a byproduct of the zinc smelting process, so the families of the victims attempted to sue U.S. Steel. The company did pay a settlement (which added up to a piddling amount after legal fees), but never took responsibility for the deaths. Ten years after the smog, mortality rates in Donora were still much higher than those in surrounding communities.

The Donora Smog helped raise awareness of air pollution and its deadly effects on human beings and has even been credited with jump-starting the clean air movement in the United States.

The Donora Smog Museum opened its doors on Oct. 20, 2008, providing a wealth of resources for those interested in the mill town’s history and the smog that devastated its population. For those who won’t be in the Monongehela River valley any time soon, a number of the museum’s photographs have been archived online.